1. Field of Invention
The invention relates to a circuit board with at least one rigid area and at least one flexible area, with a rigid individual layer which is provided with printed conductors or is copper-clad on one or both sides, with an adhesive medium and with at least one copper foil, the adhesive medium having recesses in the flexible area. In addition, the invention relates to a process for producing such a rigid-flexible circuit board.
2. Description of Related Art
Printed electrical circuits have been used for decades, for example, in electrical devices and in motor vehicles for electronic control. Usually the circuit boards are rigid, and on the one hand, connect discrete components and highly integrated modules electrically to one another, and on the other hand, act as carriers of them. Circuit boards generally consist of one or more individual layers of glass fiber-reinforced, cured epoxy resin boards which are copper-clad on one or both sides for the formation of printed circuits or conductive patterns. For multilayer circuit boards, the individual planes or the printed circuits located on the individual layers are electrically connected to one another by plated holes in the circuit board.
In addition to rigid circuit boards, for about 30 years printed circuits have also been used which have rigid and flexible areas next to one another, so-called rigid-flexible circuit boards. By providing flexible areas, a large number of rigid circuit boards can be mechanically and electrically connected to one another in almost any desired three-dimensional arrangement without plug connectors or wiring. Moreover, the flexible areas make it possible to “fold” several rigid circuit board areas on top of one another such that a large circuit board area and thus also a host of discrete components located on the circuit boards can be accommodated in a relatively small space. The flexible areas normally consist of thin polyimide films which are likewise copper-clad on one side or both sides.
These rigid-flexible circuit boards are usually built from rigid and flexible individual layers which lie on top of one another, which extend over the entire circuit, and which are cemented to one another using an adhesive medium and pressed (EP 0 408 773 B1). There are inflexible (for example, glass fiber-reinforced epoxy resin) and flexible insulation carriers (for example, polyimide film) with copper cladding on one or both sides into which the printed conductors are etched. The shape of the rigid layers fixes the rigid part of the circuit boards. The flexible areas of the circuit boards are thus produced by removing a piece of the rigid layers in these areas in several process steps.
Both a liquid and a free-flowing cement which is applied for example by screen printing and also a film with adhesive properties can be used as the adhesive medium. In practice, in addition to pure adhesive films, especially so-called prepregs are used. A prepreg consists of a resin-impregnated glass fiber fabric, the resin not being completely polymerized. Under pressure and heat the resin liquefies and as it subsequently cures, it causes cementing to the bordering individual layers. In addition to these “normal” prepregs, there are also so-called no-flow prepregs in which flowability is reduced. In addition, so-called composite films can also be used as the adhesive medium; they consists of a flexible plastic film which is provided with adhesive on both sides. Within the framework of this invention, both cements and also the above described films—adhesive films, prepregs, no-flow prepregs and composite films—are to be included in the concept of adhesive medium.
EP 0 408 773 B1 discloses a process for producing rigid-flexible circuit boards in which before lamination of the individual layers into the overall circuit a piece is punched out of the rigid individual layer and is arranged according to the size and the desired position of the flexible area. This piece of the rigid individual layer is then inserted again into the individual layer to fit as closely as possible, the composite film in the area of the circuit board which is desired to be flexible having a recess so that the piece of the individual layer which had been punched out beforehand and inserted again is not cemented to the composite film in the following process steps. After lamination, the piece must be hermetically covered during the continuing production process in order to prevent plating on the flexible area and the transition to the rigid area. This covering is complicated and expensive, and in spite of great care damage can occur which leads to irreparable faults so that the affected circuit board can no longer be used.
Another process for producing rigid-flexible circuit boards is known from German Patent Application DE-AS 26 57 212. In this process, before pressing the individual layers, a groove (pre-groove) is made in the rigid outer layers on the side facing the flexible inside layer along the separating line of the rigid and flexible area of the circuit board, the groove depth being selected such that the outside of the rigid layers remains undamaged. The composite film using which the individual layers are cemented is cut out over the flexible area of the circuit so that cementing of the rigid outer layer over the future flexible area does not take place. In addition, separating films are often inserted which prevent flow of the composite films during pressing. After cementing the individual layers and after forming the conductive patterns on the outer layers, to produce the flexible area another groove (main groove) is then milled from the outer side of the rigid layer along the separating line of the rigid and flexible area of the circuit, this groove and the pre-groove which had already been made beforehand being aligned to one another so that the milled-out piece can be removed from the rigid individual layer.
Regardless of the manner in which the piece of the rigid individual layer is removed from the area of the circuit board which is desired to be flexible, the rigid-flexible circuit board known from EP 0 408 773 B1 which underlies the invention has the disadvantage that a relatively expensive flexible individual layer (polyimide film) is used over the entire surface and is needed actually only for the comparatively small flexible area. This leads to up to 75% or more of the expensive polyimide film being “wasted”, depending on the shape of the circuit board.
Generally, rigid-flexible circuit boards which have an area of a few 10 cm2 to a few 100 cm2 are not produced separately, but a plurality of such circuit boards are located next to one another in a rigid plate which is generally called a panel. This panel typically has a square or rectangular surface with side lengths of for example 40 to 80 cm. Typically 20 to 30 completed circuit boards can be punched out of such a panel. The advantage of using these panels compared to using individual circuit boards in production is that the panels can be clamped more easily on a work table and several circuit boards can be produced at the same time with one clamping.
Since in these panels the flexible individual layer generally likewise has the dimensions of the panel, in this way the proportion of “wasted” expensive polyimide film is even increased since the polyimide film is also present in the intermediate areas between the individual circuit boards. These intermediate pieces are thrown away after the circuit boards are punched out of the panels.
For the aforementioned reasons it has been repeatedly suggested that the relatively expensive polyimide film which however has proven effective in practice be replaced by other, more economical flexible individual layers. German Patent DE 41 03 375 C1 discloses a rigid-flexible circuit board in which instead of a separate composite film and a separate polyimide film only one layer of a special flow material prepreg or a paper prepreg be used. This flow material prepreg or paper prepreg is placed directly on the entire rigid individual layer on the circuit side over the entire surface. Then likewise a copper foil is applied to the special prepreg over its entire area. To prevent the piece of the rigid individual layer which is to be removed from sticking to the special prepreg, a separating layer is applied to the side of the piece facing the prepreg. A similar rigid-flexible circuit board is known from German Patent DE 42 06 746 C1 in which, likewise, instead of an expensive flexible individual layer, a more economical thin prepreg is used which can also be bent after curing. In this circuit board, in the area which is desired to be flexible, an insulating film is also placed between the rigid individual layer and the prepreg before pressing.
German Patent Application DE 41 31 935 A1 describes a rigid-flexible circuit board which consists solely of an inherently rigid circuit board material so that expensive polyimide film has been completely abandoned. In this known circuit board which, however, withstands only a limited number of bending stresses, the rigid circuit board material in the area which is desired to be flexible has a much smaller thickness. This circuit board which can be produced relatively easily and economically is limited in its range of application, on the one hand, by the only limited flexibility, on the other hand, it is not suited for more complex circuits with several planes of printed conductors.
Finally, German Patent DE 40 03 345 C1 discloses a rigid-flexible circuit board which has a flexible polyimide film essentially only in the area which is desired to be flexible. Here, the polyimide film on the side facing the copper foil is provided with a flexible adhesive layer and on the side facing the rigid individual layer is provided with two thin adhesive strips which are located on the edge. The width and the location of the thin adhesive strips must be chosen such that the piece of the rigid individual layer which is to be removed is prevented from sticking to the adhesive strips. Since the thin adhesive strips often have a width of only 1 mm or less, the prefabrication of the polyimide film with the adhesive strips on one side and the adhesive layer on the other side is relatively time-consuming. Moreover, it is also necessary for the polyimide film which has been prepared in this way to be positioned very exactly on the rigid individual layer, since otherwise the piece of the rigid individual layer which is to be removed can likewise stick.